Distinguish between WebM-VP8 and WebM-VP9

Partial support means WebM-VP8 is supported.
Full support means all versions (WebM VP8
and WebM VP9) are supported.

VP9 was introduced in FF28:
https://bugzilla.mozilla.org/show_bug.cgi?id=833023
VP9 was introduced in Chromium25:
http://www.omgchrome.com/google-chrome-hits-25/
VP9 works for Opera 16, 17, 18 and 19
(no reference found, tested locally)